Video quality losses may be caused by various events, for example, by lossy compression and transmission errors and they may be perceived by human eyes as various types of visual artifacts. For example, blockiness, ringing, and blurriness are typical artifacts caused by lossy compression.
On the other hand, different types of artifacts may be perceived when the video quality is degraded by transmission errors. For example, when a packet loss is detected at the transport layer, a decoder may apply error concealment in order to reduce the strength of visual artifacts. Artifacts may still be perceived after error concealment, and we denote the remaining artifacts as channel artifacts. In another example, when a reference frame is entirely lost, a decoder may freeze decoding and repeats the previously correctly decoded picture until a frame without referring to the lost frame is correctly received, thus causing a visual pause. We denote such a visual pause as a freezing artifact. The freezing artifact may also be caused by buffer underflow. For example, when there is a network delay, a frame may not be available yet at a scheduled display time (i.e., the buffer underflows) and the display pauses until the frame becomes available.